This invention relates in general to the field of three-dimensional (3D) volumetric display. More specifically, it relates to correcting image position error for volumetric 3D displays based on moving screen with a rotary mechanism.
One category of volumetric 3D (V3D) displays generates V3D images by moving a screen to sweep a volume and projecting 2D profile images on the screen. V3D images thus form in the swept volume by after-mage effect. The screen motion can in general be rotating or reciprocating. A mechanism of rotational nature is generally applied to drive the motion.
For rotating screen approaches, the screen generally rotates about an axis parallel to and passing through the screen surface. Tsao U.S. Pat. No. 6,765,566 B1, which is incorporated herein by reference, describes an example. FIG. 13 illustrates a V3D display based on a rotating screen with an optical interfacing unit. It has three major portions: a rotating screen unit 1330; a high frame rate image projection system 1310; and a optical interfacing mechanism 1320, which relays the optical image projected from the image projector onto the screen for displaying, while keeping the size, orientation and focus of the projected image invariant as the screen rotates. The preferred optical interfacing mechanism is an optical image rotator rotating at half the speed of the screen. FIG. 12 illustrates an example of an optical rotator as the optical interfacing unit. The screen unit comprises a screen 1331, a central reflector 1332 and a side reflector 1333, which all rotate in unison about a common axis 1301. The projection path is from the projector 1310 to the interfacing unit 1320, then to the central reflector and then to the side reflector and then to the screen. Additional descriptions of this type of V3D display can found in Tsao et al., U.S. Pat. No. 5,754,147, 1998; Tsao, U.S. Pat. No. 5,954,414, 1999 and Tsao, U.S. Pat. No. 6,302,542 B1, 2001, which are incorporated herein by reference.
Another type of rotating screen systems does not have an optical interfacing unit rotating at ½ screen speed. Examples of this type can be found in:
1. Aviation Week, “New Display Gives Realistic 3-D Effect”, Aviation Week, Oct. 31, 1960
2. Batchko, R. G. “Rotating Flat Screen Fully Addressable Volume Display System”, U.S. Pat. No. 5,148,310, 1992
3. Shimada, S. “Rotating Screen Picture Display Apparatus”, U.S. Pat. No. 5,537,251, 1996
4. R. K. Dorval et. al., “Volumetric Three-Dimensional Display System”, U.S. Pat. No. 6,554,430, 2003
The above documents are incorporated herein for this invention by reference. In general, this type of systems is similar to that of FIG. 13 except that the interfacing unit 1320 does not exist and the projector projects to the central reflector directly. Without the interfacing unit, the projected image frame thus rotates with respect to the screen surface as the screen rotates.
For reciprocating screen approaches, Tsao U.S. Pat. No. 6,765,566 B1 describes a system with a screen that reciprocates by a rotary motion. (See FIG. 20 of the referred patent) In principle, the screen revolves about an axis while keeping its surface always facing a fixed direction, and the screen sweeps through space and defines a display volume. For convenience, this is called “Rotary Reciprocating mechanism”. In Tsao U.S. Pat. No. 6,302,542, a volumetric 3D display with a Rotary Reciprocating screen and a Rotary Reciprocating reflector serving as a linear interfacing unit is described. (See FIGS. 2a, 4b and 5b and the specification of the referred patent) The Rotary Reciprocating reflector reciprocates by a similar Rotary Reciprocating mechanism in synchronization with the screen but at a speed of ½ of the speed of the screen. FIG. 15 illustrates such a V3D display system, in side view. It has three major portions: a rotary reciprocating screen unit 1511 (comprising a screen 11 mounted on rotary arms 1522); a high frame rate image projection system 15; and an optical interfacing mechanism 13 (comprising a single reflector 1321 mounted on rotary arms 1322). A stationary reflector 1502 folds the projection path. The set of rotary arms 1322 and the set of rotary arm 1522 rotate synchronously. The figure shows the screen at the top position. When the screen rotates to the bottom position 11A, the interfacing reflector rotates to position 1321A. The projection path is from the projector 15 to the interfacing reflector 1321, then to the folding reflector 1502 and then to the screen. This projection path length is kept constant as the screen and the interfacing reflector rotate. Projection path 402 strikes the interfacing reflector 1321 at an oblique angle, therefore the sides of the resulted display volume 12 are of the shape of parallelogram. The high-frame-rate projector projects a set of 2D image frames onto the moving screen. The moving screen moves and therefore distributes the 2D image frames to corresponding positions in the space swept by the screen. Together, the spatially distributed 2D images form a volumetric 3D image.
All three types of systems, the rotating screen system with an interfacing unit the rotating screen system without an interfacing unit and the Rotary Reciprocating system, comprises rotational mechanisms. If opto-mechanical alignment in the mechanism is perfect and all dimensional tolerance is zero, then the center of projected image frame on the screen always falls on the theoretical position as the mechanism rotates. For the first two types of system, once the center of the projected image frame is aligned to the center of the rotating screen, the two centers always coincide as the screen rotates. That is, the theoretical position of the center of projected image frame on screen is the center of the screen. For the Rotary Reciprocating system, due to the motion of the screen, the theoretical position of the center of the projected image frame on screen is a straight line track falling on one of the center lines of the screen. However, since alignment and dimension of optical and mechanical elements have inevitable error and tolerance, the center of projected image frame on screen can deviate away from the theoretical position as the screen rotates.
A volumetric 3D image is formed by slices of frame images and frame images are first generated by computation based on assumption of ideal mechanism (i.e. no alignment or dimensional error). Therefore, if position error of the center of projected image frame (i.e. deviation with respect to the theoretical position) is not corrected, the volumetric 3D image will appear distorted. It is further noted that “jumping” or “jittering” of displayed volumetric 3D image can occur due to this position error. This is because one screen rotation covers two image volumes (since ½ rotations of the screen sweeps one volume), and the positions of frames relative to the screen in two successive volumes do not coincide due to this position error. The position of a V3D image therefore shifts from one volume to the next.